Memory devices reside in computers and many electronic products to store data. A typical memory device has many memory cells, each holding a charge that represents a value of a bit of data.
Some memory devices hold the charge in a capacitor of each memory cell. The charge in the capacitor leaks overtime. Therefore, some of these memory devices have refresh cycles to frequently refresh the charge to maintain its original value to keep the data valid.
In some memory devices, the charge in the capacitor of one memory cell leaks to the substrate or to an adjacent capacitor of another memory cell. This leakage reduces the retention time of the memory cell and may create invalid data. Isolation techniques have been designed to isolate adjacent memory cells to extend the retention time of the charge in the capacitor and to reduce the number of refresh cycles:
In some cases, these isolation techniques provide inadequate isolation. Thus, the number of the refresh cycles is increased. Increasing the number of refresh cycles wastes power and reduces the time that the valid data is available.
Further, some memory devices use a double-row redundancy method, in which two redundant rows of memory cells are used when a defect occurs in one of the rows; one redundant row replaces the row with the defect and the other redundant row replaces the adjacent row although the adjacent row has no defect. This double-row redundancy method is used because the isolation devices in these memory devices may not provide enough insolation between adjacent rows.